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The pivotal link between ACE2 deficiency and SARS-CoV-2 infection

  • Paolo Verdecchia
    Correspondence
    Corresponding author: Paolo Verdecchia, M.D., F.E.S.C., F.A.C.C., Fondazione Umbra Cuore e Ipertensione-ONLUS, Struttura Complessa di Cardiologia, Ospedale S. Maria della Misericordia, Perugia.
    Affiliations
    Fondazione Umbra Cuore e Ipertensione-ONLUS and Struttura Complessa di Cardiologia, Ospedale S. Maria della Misericordia, Perugia.
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  • Claudio Cavallini
    Affiliations
    Fondazione Umbra Cuore e Ipertensione-ONLUS and Struttura Complessa di Cardiologia, Ospedale S. Maria della Misericordia, Perugia.
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  • Antonio Spanevello
    Affiliations
    Dipartimento di Medicina e Chirurgia, Università degli Studi dell'Insubria, Varese

    Dipartimento di Medicina e Riabilitazione Cardio-Respiratoria, Istituti Clinici Scientici Maugeri, IRCCS Tradate (VA)
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  • Fabio Angeli
    Affiliations
    Dipartimento di Medicina e Chirurgia, Università degli Studi dell'Insubria, Varese

    Dipartimento di Medicina e Riabilitazione Cardio-Respiratoria, Istituti Clinici Scientici Maugeri, IRCCS Tradate (VA)
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Published:April 20, 2020DOI:https://doi.org/10.1016/j.ejim.2020.04.037

      Abstract

      Angiotensin converting enzyme-2 (ACE2) receptors mediate the entry into the cell of three strains of coronavirus: SARS-CoV, NL63 and SARS-CoV-2. ACE2 receptors are ubiquitous and widely expressed in the heart, vessels, gut, lung (particularly in type 2 pneumocytes and macrophages), kidney, testis and brain. ACE2 is mostly bound to cell membranes and only scarcely present in the circulation in a soluble form. An important salutary function of membrane-bound and soluble ACE2 is the degradation of angiotensin II to angiotensin1-7. Consequently, ACE2 receptors limit several detrimental effects resulting from binding of angiotensin II to AT1 receptors, which include vasoconstriction, enhanced inflammation and thrombosis. The increased generation of angiotensin1-7 also triggers counter-regulatory protective effects through binding to G-protein coupled Mas receptors. Unfortunately, the entry of SARS-CoV2 into the cells through membrane fusion markedly down-regulates ACE2 receptors, with loss of the catalytic effect of these receptors at the external site of the membrane. Increased pulmonary inflammation and coagulation have been reported as unwanted effects of enhanced and unopposed angiotensin II effects via the ACE→Angiotensin II→AT1 receptor axis. Clinical reports of patients infected with SARS-CoV-2 show that several features associated with infection and severity of the disease (i.e., older age, hypertension, diabetes, cardiovascular disease) share a variable degree of ACE2 deficiency. We suggest that ACE2 down-regulation induced by viral invasion may be especially detrimental in people with baseline ACE2 deficiency associated with the above conditions. The additional ACE2 deficiency after viral invasion might amplify the dysregulation between the ‘adverse’ ACE→Angiotensin II→AT1 receptor axis and the ‘protective’ ACE2→Angiotensin1-7→Mas receptor axis. In the lungs, such dysregulation would favor the progression of inflammatory and thrombotic processes triggered by local angiotensin II hyperactivity unopposed by angiotensin1-7. In this setting, recombinant ACE2, angiotensin1-7 and angiotensin II type 1 receptor blockers could be promising therapeutic approaches in patients with SARS-CoV-2 infection.

      Keywords

      Abbreviations:

      ADAM17 (disintegrin and metalloproteinase 17), ACE (angiotensin-converting enzyme), ACE2 (angiotensin-converting enzyme 2), COVID-19 (2019 novel coronavirus disease), DABK (des-Arg9 bradykinin), IL (interleukin), NL63 (human coronavirus NL63), RAAS (renin-angiotensin-aldosterone system), SARS (severe acute respiratory syndrome), SARS-CoV (severe acute respiratory syndrome coronavirus), SARS-CoV-2 (severe acute respiratory syndrome novel coronavirus), TMPRSS2 (transmembrane protease serine 2)

      1. Introduction

      According to the Center for Systems Science and Engineering at The Johns Hopkins University, as of April 12, 2020, the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) pandemic caused 108,867 deaths worldwide, with a total of 1,777,666 infected people (https://coronavirus.jhu.edu/map.html). Just to make a comparison, the SARS-Cov virus in years 2002-2003 infested about 8,500 people in 27 countries and caused 866 deaths. [
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      The tremendous impact of SARS-Cov-2 infection and the paucity or lack of established therapeutic measures is generating basic and clinical studies to explore the mechanisms of viral entry into the human body and the subsequent pathophysiological and therapeutic implications.
      The present review discusses the role of angiotensin converting enzyme 2 (ACE2) receptors which are not only the door through which the virus enters into cells, [
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      The entry of SARS-CoV-2 into cells is mediated by the efficient binding of the spike (S) viral protein, a 1273 amino acid long protein which belongs to the viral envelope and protrudes outwards with a ‘corona’ like appearance, to the angiotensin converting enzyme 2 (ACE2) receptors. [
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      Figure 1
      Figure 1Counter-regulatory effects of angiotensin1-7 on angiotensin II.
      ACE2 mediates the cell entry of three strains of coronavirus: SARS-CoV, NL63 and SARS-CoV-2. [
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      3. Site of ACE2 receptors

      ACE2 genes map to the X chromosome, [
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      The disintegrin and metalloproteinase 17 (ADAM17), upregulated by angiotensin II through its type 1 receptors (AT1 receptors), cleaves the membrane-anchored ACE2, thereby releasing a circulating active form of ACE2 with loss of the catalytically activity of the remaining part of the enzyme anchored to membrane. [
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      4. ACE2: angels or devils?

      In the current SARS-CoV-2 pandemic, ACE2 receptors can be considered ‘devils’, being the ‘entry door’ for the virus. Evidence about this phenomenon is now strong and convincing. Hoffmann et al [
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      In the same time, however, ACE2 receptors exert salutary biological functions that turn them as ‘angels’ under several aspects. A pivotal protective function of ACE2 is the degradation of angiotensin II to angiotensin1-7., although ACE2 is able to metabolize other biological peptides including (des-Arg9)-bradykinin. [
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      ACE2: from vasopeptidase to SARS virus receptor.
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      4.1 ACE2→Angiotensin1-7→Mas receptor axis and the lung

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      4.2 ACE2→Angiotensin1-7→Mas receptor axis and thrombosis

      The ACE2→Angiotensin1-7→Mas receptor axis exerts anti-thrombotic effects [
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      • Kucharewicz I
      • Pawlak R
      • Matys T
      • Pawlak D
      • Buczko W
      Antithrombotic effect of captopril and losartan is mediated by angiotensin-(1-7).
      ,
      • Pai WY
      • Lo WY
      • Hsu T
      • Peng CT
      • Wang HJ
      Angiotensin-(1-7) Inhibits Thrombin-Induced Endothelial Phenotypic Changes and Reactive Oxygen Species Production via NADPH Oxidase 5 Downregulation.
      ]. Mas receptors are expressed on platelets. [
      • Fraga-Silva RA
      • Costa-Fraga FP
      • De Sousa FB
      • Alenina N
      • Bader M
      • Sinisterra RD
      • Santos RA
      An orally active formulation of angiotensin-(1-7) produces an antithrombotic effect.
      ] Stimulation of Mas receptors by angiotensin1-7 increases prostacyclin and NO release. [
      • Fang C
      • Stavrou E
      • Schmaier AA
      • Grobe N
      • Morris M
      • Chen A
      • Nieman MT
      • Adams GN
      • LaRusch G
      • Zhou Y
      • Bilodeau ML
      • Mahdi F
      • Warnock M
      • Schmaier AH
      Angiotensin 1-7 and Mas decrease thrombosis in Bdkrb2-/- mice by increasing NO and prostacyclin to reduce platelet spreading and glycoprotein VI activation.
      ,
      • Fraga-Silva RA
      • Pinheiro SV
      • Goncalves AC
      • Alenina N
      • Bader M
      • Santos RA
      The antithrombotic effect of angiotensin-(1-7) involves mas-mediated NO release from platelets.
      Animals knockout for Mas receptors have a shorter bleeding time and increased size of thrombi. [
      • Fraga-Silva RA
      • Pinheiro SV
      • Goncalves AC
      • Alenina N
      • Bader M
      • Santos RA
      The antithrombotic effect of angiotensin-(1-7) involves mas-mediated NO release from platelets.
      ] In these animals, administration of angiotensin1-7 induces a marked antithrombotic effect which is directly related to the plasma levels of angiotensin1-7 [
      • Fraga-Silva RA
      • Costa-Fraga FP
      • De Sousa FB
      • Alenina N
      • Bader M
      • Sinisterra RD
      • Santos RA
      An orally active formulation of angiotensin-(1-7) produces an antithrombotic effect.
      ] and is inhibited by A-779, an antagonist of Mas receptors. [
      • Fang C
      • Stavrou E
      • Schmaier AA
      • Grobe N
      • Morris M
      • Chen A
      • Nieman MT
      • Adams GN
      • LaRusch G
      • Zhou Y
      • Bilodeau ML
      • Mahdi F
      • Warnock M
      • Schmaier AH
      Angiotensin 1-7 and Mas decrease thrombosis in Bdkrb2-/- mice by increasing NO and prostacyclin to reduce platelet spreading and glycoprotein VI activation.
      ] Thus, angiotensin1-7 plays an important role in opposing the pro-thrombotic and pro-inflammatory effects of angiotensin II. [
      • Liang B
      • Wang X
      • Zhang N
      • Yang H
      • Bai R
      • Liu M
      • Bian Y
      • Xiao C
      • Yang Z
      Angiotensin-(1-7) Attenuates Angiotensin II-Induced ICAM-1, VCAM-1, and MCP-1 Expression via the MAS Receptor Through Suppression of P38 and NF-kappaB Pathways in HUVECs.
      ,
      • Mehta PK
      • Griendling KK
      Angiotensin II cell signaling: physiological and pathological effects in the cardiovascular system.

      4.3 ACE2→Angiotensin1-7→Mas receptor axis and the endocrine system

      The ACE2→Angiotensin1-7→Mas receptor axis is well expressed in the pancreas where it improves insulin secretion possibly by improving peri-insular blood flow and inhibiting fibrosis as a result of increased NO release. [
      • Santos RAS
      • Sampaio WO
      • Alzamora AC
      • Motta-Santos D
      • Alenina N
      • Bader M
      • Campagnole-Santos MJ
      The ACE2/Angiotensin-(1-7)/MAS Axis of the Renin-Angiotensin System: Focus on Angiotensin-(1-7).
      ,
      • Yuan L
      • Li Y
      • Li G
      • Song Y
      • Gong X
      Ang(1-7) treatment attenuates beta-cell dysfunction by improving pancreatic microcirculation in a rat model of Type 2 diabetes.
      ACE2 receptors are also expressed in the adipose tissue [
      • Gembardt F
      • Sterner-Kock A
      • Imboden H
      • Spalteholz M
      • Reibitz F
      • Schultheiss HP
      • Siems WE
      • Walther T
      Organ-specific distribution of ACE2 mRNA and correlating peptidase activity in rodents.
      ,
      • Gupte M
      • Boustany-Kari CM
      • Bharadwaj K
      • Police S
      • Thatcher S
      • Gong MC
      • English VL
      • Cassis LA
      ACE2 is expressed in mouse adipocytes and regulated by a high-fat diet.
      and a reduction of ACE2 has been noted in the adipose tissue of obese animals [
      • Gupte M
      • Boustany-Kari CM
      • Bharadwaj K
      • Police S
      • Thatcher S
      • Gong MC
      • English VL
      • Cassis LA
      ACE2 is expressed in mouse adipocytes and regulated by a high-fat diet.
      ] In animal experiments, diets rich of fats decreased ACE2 activity and angiotensin1-7, and increased angiotensin II and blood pressure levels in male, but not in female, animals and these reactions were inhibited by AT1 blockade with losartan. [
      • Gupte M
      • Thatcher SE
      • Boustany-Kari CM
      • Shoemaker R
      • Yiannikouris F
      • Zhang X
      • Karounos M
      • Cassis LA
      Angiotensin converting enzyme 2 contributes to sex differences in the development of obesity hypertension in C57BL/6 mice.
      ] After ovariectomy, female animals showed similar reactions as in males. [
      • Gupte M
      • Thatcher SE
      • Boustany-Kari CM
      • Shoemaker R
      • Yiannikouris F
      • Zhang X
      • Karounos M
      • Cassis LA
      Angiotensin converting enzyme 2 contributes to sex differences in the development of obesity hypertension in C57BL/6 mice.
      ] These data suggest that ACE2 deficiency may favor obesity-induced hypertension. [
      • Gupte M
      • Thatcher SE
      • Boustany-Kari CM
      • Shoemaker R
      • Yiannikouris F
      • Zhang X
      • Karounos M
      • Cassis LA
      Angiotensin converting enzyme 2 contributes to sex differences in the development of obesity hypertension in C57BL/6 mice.
      ] ACE2 is also expressed in the cardiac adipocytes. [
      • Patel VB
      • Basu R
      • Oudit GY
      ACE2/Ang 1-7 axis: A critical regulator of epicardial adipose tissue inflammation and cardiac dysfunction in obesity.
      ] Obese patients with heart failure have an increased amount of epicardial adipose tissue [
      • Patel VB
      • Basu R
      • Oudit GY
      ACE2/Ang 1-7 axis: A critical regulator of epicardial adipose tissue inflammation and cardiac dysfunction in obesity.
      ] and it has been suggested that ACE2 deficiency can induce heart failure with preserved ejection fraction in animals. [
      • Patel VB
      • Mori J
      • McLean BA
      • Basu R
      • Das SK
      • Ramprasath T
      • Parajuli N
      • Penninger JM
      • Grant MB
      • Lopaschuk GD
      • Oudit GY
      ACE2 Deficiency Worsens Epicardial Adipose Tissue Inflammation and Cardiac Dysfunction in Response to Diet-Induced Obesity.
      ] This phenomenon has been attributed to adipose tissue inflammation through local activation of macrophages, which possess AT1 receptors on their cellular membrane. [
      • Yamamoto S
      • Yancey PG
      • Zuo Y
      • Ma LJ
      • Kaseda R
      • Fogo AB
      • Ichikawa I
      • Linton MF
      • Fazio S
      • Kon V
      Macrophage polarization by angiotensin II-type 1 receptor aggravates renal injury-acceleration of atherosclerosis.
      ]

      5. What does it happen to ACE2 after SARS-Cov binding?

      SARS-Cov and SARS-CoV2 bind to ACE2 receptors, with the subsequent membrane fusion and virus entry into the cell, leads to down-regulation of these receptors. [
      • Zhang H
      • Penninger JM
      • Li Y
      • Zhong N
      • Slutsky AS
      Angiotensin-converting enzyme 2 (ACE2) as a SARS-CoV-2 receptor: molecular mechanisms and potential therapeutic target.
      ,
      • Kuba K
      • Imai Y
      • Rao S
      • Gao H
      • Guo F
      • Guan B
      • Huan Y
      • Yang P
      • Zhang Y
      • Deng W
      • Bao L
      • Zhang B
      • Liu G
      • Wang Z
      • Chappell M
      • Liu Y
      • Zheng D
      • Leibbrandt A
      • Wada T
      • Slutsky AS
      • Liu D
      • Qin C
      • Jiang C
      • Penninger JM
      A crucial role of angiotensin converting enzyme 2 (ACE2) in SARS coronavirus-induced lung injury.
      ,
      • Imai Y
      • Kuba K
      • Rao S
      • Huan Y
      • Guo F
      • Guan B
      • Yang P
      • Sarao R
      • Wada T
      • Leong-Poi H
      • Crackower MA
      • Fukamizu A
      • Hui CC
      • Hein L
      • Uhlig S
      • Slutsky AS
      • Jiang C
      • Penninger JM
      Angiotensin-converting enzyme 2 protects from severe acute lung failure.
      In other terms, the virus appears to entry into the cell along with the membrane receptor, which is functionally removed from the external site of the membrane.
      As a result, the ACE→Angiotensin II→Mas receptor axis is markedly attenuated, with amplification of the ACE→Angiotensin II→AT1 receptor axis.

      5.1 Pulmonary implications of ACE2 down-regulation

      Since the pulmonary inflammation and the resulting Acute Respiratory Distress Syndrome (ARDS) are potentially deadly complications of SARS-CoV and SARS-CoV-2, studies addressing the lung complications of ACE2 down-regulation are of outmost importance. Studies using different models of lung injury showed that the down-regulation of ACE2 receptors triggers important inflammatory lesions in the respiratory tree (alveolar wall thickening, edema, infiltrates of inflammatory cells, bleeding) which appear to be mediated by angiotensin II. [
      • Kuba K
      • Imai Y
      • Rao S
      • Gao H
      • Guo F
      • Guan B
      • Huan Y
      • Yang P
      • Zhang Y
      • Deng W
      • Bao L
      • Zhang B
      • Liu G
      • Wang Z
      • Chappell M
      • Liu Y
      • Zheng D
      • Leibbrandt A
      • Wada T
      • Slutsky AS
      • Liu D
      • Qin C
      • Jiang C
      • Penninger JM
      A crucial role of angiotensin converting enzyme 2 (ACE2) in SARS coronavirus-induced lung injury.
      ,
      • Imai Y
      • Kuba K
      • Rao S
      • Huan Y
      • Guo F
      • Guan B
      • Yang P
      • Sarao R
      • Wada T
      • Leong-Poi H
      • Crackower MA
      • Fukamizu A
      • Hui CC
      • Hein L
      • Uhlig S
      • Slutsky AS
      • Jiang C
      • Penninger JM
      Angiotensin-converting enzyme 2 protects from severe acute lung failure.
      ,
      • Hung YH
      • Hsieh WY
      • Hsieh JS
      • Liu FC
      • Tsai CH
      • Lu LC
      • Huang CY
      • Wu CL
      • Lin CS
      Alternative Roles of STAT3 and MAPK Signaling Pathways in the MMPs Activation and Progression of Lung Injury Induced by Cigarette Smoke Exposure in ACE2 Knockout Mice.
      ,
      • Lin CI
      • Tsai CH
      • Sun YL
      • Hsieh WY
      • Lin YC
      • Chen CY
      • Lin CS
      Instillation of particulate matter 2.5 induced acute lung injury and attenuated the injury recovery in ACE2 knockout mice.
      Tracheal instillation of cigarette smoke, [
      • Hung YH
      • Hsieh WY
      • Hsieh JS
      • Liu FC
      • Tsai CH
      • Lu LC
      • Huang CY
      • Wu CL
      • Lin CS
      Alternative Roles of STAT3 and MAPK Signaling Pathways in the MMPs Activation and Progression of Lung Injury Induced by Cigarette Smoke Exposure in ACE2 Knockout Mice.
      ] or particulate matter of aerodynamic diameter of less than 2,5 ɥm, [
      • Lin CI
      • Tsai CH
      • Sun YL
      • Hsieh WY
      • Lin YC
      • Chen CY
      • Lin CS
      Instillation of particulate matter 2.5 induced acute lung injury and attenuated the injury recovery in ACE2 knockout mice.
      ] induces acute lung injury with release of inflammatory cytokines IL-6, TNF-α and TGF-β1 and increased expression of ACE, consistent with ACE→Angiotensin II→AT1 receptor axis over-activity. [
      • Lin CI
      • Tsai CH
      • Sun YL
      • Hsieh WY
      • Lin YC
      • Chen CY
      • Lin CS
      Instillation of particulate matter 2.5 induced acute lung injury and attenuated the injury recovery in ACE2 knockout mice.
      ] These reactions are increased in ACE2 knockout mice. [
      • Lin CI
      • Tsai CH
      • Sun YL
      • Hsieh WY
      • Lin YC
      • Chen CY
      • Lin CS
      Instillation of particulate matter 2.5 induced acute lung injury and attenuated the injury recovery in ACE2 knockout mice.
      ] In a model of acid aspiration, which induces acute lung injury, lung inflammatory lesions were more severe and lethal in ACE2 knock-out animals. [
      • Imai Y
      • Kuba K
      • Rao S
      • Huan Y
      • Guo F
      • Guan B
      • Yang P
      • Sarao R
      • Wada T
      • Leong-Poi H
      • Crackower MA
      • Fukamizu A
      • Hui CC
      • Hein L
      • Uhlig S
      • Slutsky AS
      • Jiang C
      • Penninger JM
      Angiotensin-converting enzyme 2 protects from severe acute lung failure.
      ] In these animals, injection of recombinant ACE2 as well as AT1 receptor blockers attenuate the degree of lung injury. [
      • Imai Y
      • Kuba K
      • Rao S
      • Huan Y
      • Guo F
      • Guan B
      • Yang P
      • Sarao R
      • Wada T
      • Leong-Poi H
      • Crackower MA
      • Fukamizu A
      • Hui CC
      • Hein L
      • Uhlig S
      • Slutsky AS
      • Jiang C
      • Penninger JM
      Angiotensin-converting enzyme 2 protects from severe acute lung failure.
      ] These findings strongly suggest that ACE2 protects from lung injury induced by acid aspiration. [
      • Imai Y
      • Kuba K
      • Rao S
      • Huan Y
      • Guo F
      • Guan B
      • Yang P
      • Sarao R
      • Wada T
      • Leong-Poi H
      • Crackower MA
      • Fukamizu A
      • Hui CC
      • Hein L
      • Uhlig S
      • Slutsky AS
      • Jiang C
      • Penninger JM
      Angiotensin-converting enzyme 2 protects from severe acute lung failure.
      ]
      Notably, lung injury has been induced by the isolated spike viral protein of the SARS-Cov, the ligand for ACE2 binding, in the absence of other viral components. [
      • Kuba K
      • Imai Y
      • Rao S
      • Gao H
      • Guo F
      • Guan B
      • Huan Y
      • Yang P
      • Zhang Y
      • Deng W
      • Bao L
      • Zhang B
      • Liu G
      • Wang Z
      • Chappell M
      • Liu Y
      • Zheng D
      • Leibbrandt A
      • Wada T
      • Slutsky AS
      • Liu D
      • Qin C
      • Jiang C
      • Penninger JM
      A crucial role of angiotensin converting enzyme 2 (ACE2) in SARS coronavirus-induced lung injury.
      ] This model has the merit to investigate the impact of ACE2 down-regulation in the absence of confounding effects of viral invasion and replication. The authors found that even the isolated spike viral protein induced down-regulation of ACE2 receptors with concomitant increase of angiotensin II in the lung tissue and precipitation of severe pulmonary inflammatory lesions. [
      • Kuba K
      • Imai Y
      • Rao S
      • Gao H
      • Guo F
      • Guan B
      • Huan Y
      • Yang P
      • Zhang Y
      • Deng W
      • Bao L
      • Zhang B
      • Liu G
      • Wang Z
      • Chappell M
      • Liu Y
      • Zheng D
      • Leibbrandt A
      • Wada T
      • Slutsky AS
      • Liu D
      • Qin C
      • Jiang C
      • Penninger JM
      A crucial role of angiotensin converting enzyme 2 (ACE2) in SARS coronavirus-induced lung injury.
      ] Also in this model, AT1 receptor blockers attenuated the pulmonary lesions induced by the spike viral protein. [
      • Kuba K
      • Imai Y
      • Rao S
      • Gao H
      • Guo F
      • Guan B
      • Huan Y
      • Yang P
      • Zhang Y
      • Deng W
      • Bao L
      • Zhang B
      • Liu G
      • Wang Z
      • Chappell M
      • Liu Y
      • Zheng D
      • Leibbrandt A
      • Wada T
      • Slutsky AS
      • Liu D
      • Qin C
      • Jiang C
      • Penninger JM
      A crucial role of angiotensin converting enzyme 2 (ACE2) in SARS coronavirus-induced lung injury.
      ]
      A key point to remark is that ACE2 are mainly expressed in pneumocytes type II, small cylindrical cells which represent 5% of all pneumocytes. [
      • Hamming I
      • Timens W
      • Bulthuis ML
      • Lely AT
      • Navis G
      • van Goor H
      Tissue distribution of ACE2 protein, the functional receptor for SARS coronavirus. A first step in understanding SARS pathogenesis.
      ] Pneumocytes type 2 are responsible for the production of alveolar surfactant, and in the same time they function as ‘stem’ cells, progenitors of pneumocytes type I (95% of all pneumocytes) which are responsible of gas exchanges. [
      • Barkauskas CE
      • Cronce MJ
      • Rackley CR
      • Bowie EJ
      • Keene DR
      • Stripp BR
      • Randell SH
      • Noble PW
      • Hogan BL
      Type 2 alveolar cells are stem cells in adult lung.
      ] Therefore, the damage of pneumocytes type II due to the binding of coronavirus to ACE2 receptors is devastating for at least three reasons: 1) local unopposed ACE→Angiotensin II→AT1 receptor axis over-activity; 2) reduced production of alveolar surfactant by injured pneumocytes type II leading to reduced lung elasticity; 3) reduced repair of pneumocytes type I leading to impaired gas exchanges and fibrosis. [
      • Rivellese F
      • Prediletto E.
      ACE2 at the centre of COVID-19 from paucisymptomatic infections to severe pneumonia.
      ]

      6. Clinical characteristics of patients infected with SARS-CoV and SARS-CoV-2

      Studies from China and Italy have shown that hypertension, diabetes and history of cardiovascular disease are the most frequent comorbidities in patients infected with SARS-CoV-2. [
      • Grasselli G
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      • Iotti G
      • Latronico N
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      • Merler S
      • Natalini G
      • Piatti A
      • Ranieri MV
      • Scandroglio AM
      • Storti E
      • Cecconi M
      • Pesenti A
      • Network C-LI
      Baseline Characteristics and Outcomes of 1591 Patients Infected With SARS-CoV-2 Admitted to ICUs of the Lombardy Region, Italy.
      ,
      • Wu C
      • Chen X
      • Cai Y
      • Xia J
      • Zhou X
      • Xu S
      • Huang H
      • Zhang L
      • Zhou X
      • Du C
      • Zhang Y
      • Song J
      • Wang S
      • Chao Y
      • Yang Z
      • Xu J
      • Zhou X
      • Chen D
      • Xiong W
      • Xu L
      • Zhou F
      • Jiang J
      • Bai C
      • Zheng J
      • Song Y
      Risk Factors Associated With Acute Respiratory Distress Syndrome and Death in Patients With Coronavirus Disease 2019 Pneumonia in Wuhan, China.
      ,
      • Yang J
      • Zheng Y
      • Gou X
      • Pu K
      • Chen Z
      • Guo Q
      • Ji R
      • Wang H
      • Wang Y
      • Zhou Y
      Prevalence of comorbidities in the novel Wuhan coronavirus (COVID-19) infection: a systematic review and meta-analysis.
      ] Older age and male sex are two additional factors associated with SARS-CoV-2 infection. [
      • Grasselli G
      • Zangrillo A
      • Zanella A
      • Antonelli M
      • Cabrini L
      • Castelli A
      • Cereda D
      • Coluccello A
      • Foti G
      • Fumagalli R
      • Iotti G
      • Latronico N
      • Lorini L
      • Merler S
      • Natalini G
      • Piatti A
      • Ranieri MV
      • Scandroglio AM
      • Storti E
      • Cecconi M
      • Pesenti A
      • Network C-LI
      Baseline Characteristics and Outcomes of 1591 Patients Infected With SARS-CoV-2 Admitted to ICUs of the Lombardy Region, Italy.
      ,
      • Wu C
      • Chen X
      • Cai Y
      • Xia J
      • Zhou X
      • Xu S
      • Huang H
      • Zhang L
      • Zhou X
      • Du C
      • Zhang Y
      • Song J
      • Wang S
      • Chao Y
      • Yang Z
      • Xu J
      • Zhou X
      • Chen D
      • Xiong W
      • Xu L
      • Zhou F
      • Jiang J
      • Bai C
      • Zheng J
      • Song Y
      Risk Factors Associated With Acute Respiratory Distress Syndrome and Death in Patients With Coronavirus Disease 2019 Pneumonia in Wuhan, China.
      ,
      • Yang J
      • Zheng Y
      • Gou X
      • Pu K
      • Chen Z
      • Guo Q
      • Ji R
      • Wang H
      • Wang Y
      • Zhou Y
      Prevalence of comorbidities in the novel Wuhan coronavirus (COVID-19) infection: a systematic review and meta-analysis.
      ] A similar picture emerged a few years ago with the SARS-CoV infection. [
      • Booth CM
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      • Rose DB
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      • Derkach P
      • Ephtimios IE
      • Kitai I
      • Mederski BD
      • Shadowitz SB
      • Gold WL
      • Hawryluck LA
      • Rea E
      • Chenkin JS
      • Cescon DW
      • Poutanen SM
      • Detsky AS
      Clinical features and short-term outcomes of 144 patients with SARS in the greater Toronto area.
      ,
      • Chan JW
      • Ng CK
      • Chan YH
      • Mok TY
      • Lee S
      • Chu SY
      • Law WL
      • Lee MP
      • Li PC
      Short term outcome and risk factors for adverse clinical outcomes in adults with severe acute respiratory syndrome (SARS).
      In a study conducted in 201 patients infected with SARS-CoV-2, most patients were men (63.7% of patients), the mean age was 51 years and the most frequent comorbidities were hypertension (19.4%), diabetes (10.9%) and history of cardiovascular disease (4.0). [
      • Wu C
      • Chen X
      • Cai Y
      • Xia J
      • Zhou X
      • Xu S
      • Huang H
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      • Chao Y
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      • Xu J
      • Zhou X
      • Chen D
      • Xiong W
      • Xu L
      • Zhou F
      • Jiang J
      • Bai C
      • Zheng J
      • Song Y
      Risk Factors Associated With Acute Respiratory Distress Syndrome and Death in Patients With Coronavirus Disease 2019 Pneumonia in Wuhan, China.
      ] Notably, the patients who developed ARDS were older and had a higher prevalence of hypertension (27.4% Vs. 13.7%), diabetes (19.0% Vs. 5.1%) when compared with those who did not develop ARDS. [
      • Wu C
      • Chen X
      • Cai Y
      • Xia J
      • Zhou X
      • Xu S
      • Huang H
      • Zhang L
      • Zhou X
      • Du C
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      • Song J
      • Wang S
      • Chao Y
      • Yang Z
      • Xu J
      • Zhou X
      • Chen D
      • Xiong W
      • Xu L
      • Zhou F
      • Jiang J
      • Bai C
      • Zheng J
      • Song Y
      Risk Factors Associated With Acute Respiratory Distress Syndrome and Death in Patients With Coronavirus Disease 2019 Pneumonia in Wuhan, China.
      ] In a multivariate analysis, the factors associated with progression from ARDS syndrome to death included older age, neutrophilia and hyper-coagulation, mainly reflected by a higher D-dimer. [
      • Wu C
      • Chen X
      • Cai Y
      • Xia J
      • Zhou X
      • Xu S
      • Huang H
      • Zhang L
      • Zhou X
      • Du C
      • Zhang Y
      • Song J
      • Wang S
      • Chao Y
      • Yang Z
      • Xu J
      • Zhou X
      • Chen D
      • Xiong W
      • Xu L
      • Zhou F
      • Jiang J
      • Bai C
      • Zheng J
      • Song Y
      Risk Factors Associated With Acute Respiratory Distress Syndrome and Death in Patients With Coronavirus Disease 2019 Pneumonia in Wuhan, China.
      ] Abnormal coagulation parameters and enhanced thrombosis predict a poor prognosis in patients with SARS-CoV-2. [
      • Tang N
      • Li D
      • Wang X
      • Sun Z
      Abnormal coagulation parameters are associated with poor prognosis in patients with novel coronavirus pneumonia.
      ] A meta-analysis of 8 studies conducted in China on a total of 46,248 patients infected with SARS-CoV2 confirmed that hypertension, diabetes and history of cardiovascular disease were the most frequent comorbidities in these patients. [
      • Yang J
      • Zheng Y
      • Gou X
      • Pu K
      • Chen Z
      • Guo Q
      • Ji R
      • Wang H
      • Wang Y
      • Zhou Y
      Prevalence of comorbidities in the novel Wuhan coronavirus (COVID-19) infection: a systematic review and meta-analysis.
      ] Again, hypertension and history of cardiovascular disease were significantly more prevalent among the more severe patients. [
      • Yang J
      • Zheng Y
      • Gou X
      • Pu K
      • Chen Z
      • Guo Q
      • Ji R
      • Wang H
      • Wang Y
      • Zhou Y
      Prevalence of comorbidities in the novel Wuhan coronavirus (COVID-19) infection: a systematic review and meta-analysis.
      ]
      In a recent analysis of 1591 infected patients from Italy, the mean age of patients was 63 years, men were 82% and the prevalence of patients with hypertension, diabetes and previous cardiovascular disease was 49%, 17% and 21%, respectively. [
      • Grasselli G
      • Zangrillo A
      • Zanella A
      • Antonelli M
      • Cabrini L
      • Castelli A
      • Cereda D
      • Coluccello A
      • Foti G
      • Fumagalli R
      • Iotti G
      • Latronico N
      • Lorini L
      • Merler S
      • Natalini G
      • Piatti A
      • Ranieri MV
      • Scandroglio AM
      • Storti E
      • Cecconi M
      • Pesenti A
      • Network C-LI
      Baseline Characteristics and Outcomes of 1591 Patients Infected With SARS-CoV-2 Admitted to ICUs of the Lombardy Region, Italy.
      ] Patients with hypertension were older that those without hypertension (66 vs 62 years, p=0.005). When comparing the patients who died in the Intensive Care Unit with those who survived, the former were older and had a higher prevalence of hypertension (63% Vs. 40%, p<0.001). [
      • Grasselli G
      • Zangrillo A
      • Zanella A
      • Antonelli M
      • Cabrini L
      • Castelli A
      • Cereda D
      • Coluccello A
      • Foti G
      • Fumagalli R
      • Iotti G
      • Latronico N
      • Lorini L
      • Merler S
      • Natalini G
      • Piatti A
      • Ranieri MV
      • Scandroglio AM
      • Storti E
      • Cecconi M
      • Pesenti A
      • Network C-LI
      Baseline Characteristics and Outcomes of 1591 Patients Infected With SARS-CoV-2 Admitted to ICUs of the Lombardy Region, Italy.
      ]

      7. ACE2 deficiency: a central role in SARS-CoV-2 infection?

      It is interesting to note that several conditions associated with viral infection and severity of the disease share a variable degree of ACE2 deficiency. For example, ACE2 expression in the lungs markedly decreases with ageing, [
      • Xie X
      • Chen J
      • Wang X
      • Zhang F
      • Liu Y
      Age- and gender-related difference of ACE2 expression in rat lung.
      ] to a greater extent in men than women. [
      • Xie X
      • Chen J
      • Wang X
      • Zhang F
      • Liu Y
      Age- and gender-related difference of ACE2 expression in rat lung.
      ] Diabetes mellitus has been associated with reduced ACE2 expression, possibly as effect of glycosylation. [
      • Pal R
      • Bhansali A.
      COVID-19, Diabetes Mellitus and ACE2: The conundrum.
      ,
      • Tikellis C
      • Thomas MC
      Angiotensin-Converting Enzyme 2 (ACE2) Is a Key Modulator of the Renin Angiotensin System in Health and Disease.
      ,
      • Yamagata R
      • Nemoto W
      • Nakagawasai O
      • Takahashi K
      • Tan-No K
      Downregulation of spinal angiotensin converting enzyme 2 is involved in neuropathic pain associated with type 2 diabetes mellitus in mice.
      ] Several experimental and clinical studies indicate that ACE2 deficiency obtained through deletion or inhibition may be a causative factor for hypertension. [
      • Patel VB
      • Zhong JC
      • Grant MB
      • Oudit GY
      Role of the ACE2/Angiotensin 1-7 Axis of the Renin-Angiotensin System in Heart Failure.
      ,
      • Patel SK
      • Velkoska E
      • Freeman M
      • Wai B
      • Lancefield TF
      • Burrell LM
      From gene to protein-experimental and clinical studies of ACE2 in blood pressure control and arterial hypertension.
      Treatment with soluble recombinant ACE2 reduces the blood pressure rise provoked by angiotensin II, increases angiotensin1-7 and reduces angiotensin II. [
      • Wysocki J
      • Ye M
      • Rodriguez E
      • Gonzalez-Pacheco FR
      • Barrios C
      • Evora K
      • Schuster M
      • Loibner H
      • Brosnihan KB
      • Ferrario CM
      • Penninger JM
      • Batlle D
      Targeting the degradation of angiotensin II with recombinant angiotensin-converting enzyme 2: prevention of angiotensin II-dependent hypertension.
      ] ACE2 deficiency has been associated with exacerbation of hypertension and cardiac hypertrophy induced by angiotensin II, [
      • Zhong J
      • Basu R
      • Guo D
      • Chow FL
      • Byrns S
      • Schuster M
      • Loibner H
      • Wang XH
      • Penninger JM
      • Kassiri Z
      • Oudit GY
      Angiotensin-converting enzyme 2 suppresses pathological hypertrophy, myocardial fibrosis, and cardiac dysfunction.
      ] and maladaptive left ventricular remodeling after myocardial infarction. [
      • Kassiri Z
      • Zhong J
      • Guo D
      • Basu R
      • Wang X
      • Liu PP
      • Scholey JW
      • Penninger JM
      • Oudit GY
      Loss of angiotensin-converting enzyme 2 accelerates maladaptive left ventricular remodeling in response to myocardial infarction.
      ] Furthermore, deficiency of ACE2 enhances the susceptibility to heart failure. [
      • Patel VB
      • Zhong JC
      • Grant MB
      • Oudit GY
      Role of the ACE2/Angiotensin 1-7 Axis of the Renin-Angiotensin System in Heart Failure.
      ] A heterozygote loss of ACE2 is believed sufficient to increase the susceptibility to heart disease. [
      • Wang W
      • Patel VB
      • Parajuli N
      • Fan D
      • Basu R
      • Wang Z
      • Ramprasath T
      • Kassiri Z
      • Penninger JM
      • Oudit GY
      Heterozygote loss of ACE2 is sufficient to increase the susceptibility to heart disease.
      ]
      Given the above premises, it is tempting to speculate (Figure 2) that ACE2 deficiency may play a central role in the pathogenesis of SARS-CoV-2 infection. The down-regulation of ACE2 induced by viral invasion could be especially detrimental in individuals with baseline ACE2 deficiency due, for example, to older age, diabetes, hypertension and prior heart diseases including heart failure.
      Figure 2
      Figure 2Potential impact of ACE2 down-regulation induced by viral entry in a setting of pre-existing ACE2 deficiency.
      The possibility that a mild or moderate ACE2 deficiency may protect from viral invasion seems unlikely because of the intrinsically high affinity of SARS-CoV-2 to ACE2 receptors. [
      • Hoffmann M
      • Kleine-Weber H
      • Schroeder S
      • Kruger N
      • Herrler T
      • Erichsen S
      • Schiergens TS
      • Herrler G
      • Wu NH
      • Nitsche A
      • Muller MA
      • Drosten C
      • Pohlmann S
      SARS-CoV-2 Cell Entry Depends on ACE2 and TMPRSS2 and Is Blocked by a Clinically Proven Protease Inhibitor.
      ,
      • Walls AC
      • Park YJ
      • Tortorici MA
      • Wall A
      • McGuire AT
      • Veesler D
      Structure, Function, and Antigenicity of the SARS-CoV-2 Spike Glycoprotein.
      By contrast, in a setting of ACE2 deficiency, the ACE2 down-regulation induced by virus could amplify the imbalance between the ACE→Angiotensin II→AT1 receptor axis (adverse) and the ACE2→Angiotensin1-7I→Mas receptor axis (protective). At lung level, such dysregulation would much facilitate the progression of inflammatory and hyper-coagulation processes which share dependency upon local angiotensin II hyper-activity insufficiently opposed by angiotensin1-7. This chain of events would not deny the concomitant role of other mechanisms including an impaired immune response to initial viral invasion, or a genetic susceptibility to hyper-inflammation and thrombosis. [
      • Mehta P
      • MaAuley DF
      • Brown M
      • Sanchez E
      • Tattersall RS
      • Manson J
      COVID-19: consider cytokine storm syndromes and immunosuppression.
      ,
      • Akhmerov A
      • Marban E.
      COVID-19 and the Heart.
      In this context, administration of soluble recombinant ACE2 [
      • Batlle D
      • Wysocki J
      • Satchell K
      Soluble angiotensin-converting enzyme 2: a potential approach for coronavirus infection therapy?.
      ] or angiotensin1-7 [
      • Peiro C
      • Moncada S
      Substituting Angiotensin-(1-7) to Prevent Lung Damage in SARSCoV2 Infection?.
      ] could be promising therapeutic approaches, requiring urgent evaluation in clinical trials. Two trials of losartan as additional treatment for SARS-CoV-2 infection in hospitalized (NCT04312009) or not hospitalized (NCT04311177) patients have been announced, supported by the background of the huge adverse impact of the ACE→Angiotensin II→AT1 receptor axis over-activity in these patients.
      The possibility that ACE inhibitors and angiotensin II receptor blockers (ARBs) may be discontinued even temporarily, because these drugs appear to increase the expression of ACE2 receptors, the site of viral entry into the human organism, [
      • Ferrario CM
      • Jessup J
      • Chappell MC
      • Averill DB
      • Brosnihan KB
      • Tallant EA
      • Diz DI
      • Gallagher PE
      Effect of angiotensin-converting enzyme inhibition and angiotensin II receptor blockers on cardiac angiotensin-converting enzyme 2.
      ,
      • Gallagher PE
      • Ferrario CM
      • Tallant EA
      MAP kinase/phosphatase pathway mediates the regulation of ACE2 by angiotensin peptides.
      ,
      • Ishiyama Y
      • Gallagher PE
      • Averill DB
      • Tallant EA
      • Brosnihan KB
      • Ferrario CM
      Upregulation of angiotensin-converting enzyme 2 after myocardial infarction by blockade of angiotensin II receptors.
      ,
      • Jessup JA
      • Gallagher PE
      • Averill DB
      • Brosnihan KB
      • Tallant EA
      • Chappell MC
      • Ferrario CM
      Effect of angiotensin II blockade on a new congenic model of hypertension derived from transgenic Ren-2 rats.
      ] is actively debated. [
      • Bavishi C
      • Maddox TM
      • Messerli FH
      Coronavirus Disease 2019 (COVID-19) Infection and Renin Angiotensin System Blockers.
      ,
      • Danser AHJ
      • Epstein M
      • Batlle D
      Renin-Angiotensin System Blockers and the COVID-19 Pandemic: At Present There Is No Evidence to Abandon Renin-Angiotensin System Blockers.
      ,
      • Esler M
      • Esler D.
      Can angiotensin receptor-blocking drugs perhaps be harmful in the COVID-19 pandemic?.
      ,
      • Fang L
      • Karakiulakis G
      • TRoth M
      Are patients with hypertension and diabetes mellitus at increased risk for COVID-19 infection?.
      ,
      • Gurwitz D.
      Angiotensin receptor blockers as tentative SARS-CoV-2 therapeutics.
      ,
      • Kuster GM
      • Pfister O
      • Burkard T
      • Zhou Q
      • Twerenbold R
      • Haaf P
      • Widmer AF
      • Osswald S
      SARS-CoV2: should inhibitors of the renin-angiotensin system be withdrawn in patients with COVID-19?.
      ,
      • Vaduganathan M
      • Vardeny O
      • Michel T
      • McMurray JJV
      • Pfeffer MA
      • Solomon SD
      Renin-Angiotensin-Aldosterone System Inhibitors in Patients with Covid-19.
      ,
      • Verdecchia P
      • Angeli F
      • Reboldi G
      Angiotensin-converting enzyme inhibitors, angiotensin II receptor blockers and coronavirus.
      ,
      • Verdecchia P
      • Reboldi G
      • Cavallini C
      • Mazzotta G
      • Angeli F
      ACE-inibitori, sartani e sindrome respiratoria acuta da coronavirus 2.
      ] Several Scientific Societies and various experts in this area expressed the position that discontinuation of these drugs is not justified by evidence and could be dangerous. [
      • Bavishi C
      • Maddox TM
      • Messerli FH
      Coronavirus Disease 2019 (COVID-19) Infection and Renin Angiotensin System Blockers.
      ,
      • Danser AHJ
      • Epstein M
      • Batlle D
      Renin-Angiotensin System Blockers and the COVID-19 Pandemic: At Present There Is No Evidence to Abandon Renin-Angiotensin System Blockers.
      ,
      • Kuster GM
      • Pfister O
      • Burkard T
      • Zhou Q
      • Twerenbold R
      • Haaf P
      • Widmer AF
      • Osswald S
      SARS-CoV2: should inhibitors of the renin-angiotensin system be withdrawn in patients with COVID-19?.
      ,
      • Vaduganathan M
      • Vardeny O
      • Michel T
      • McMurray JJV
      • Pfeffer MA
      • Solomon SD
      Renin-Angiotensin-Aldosterone System Inhibitors in Patients with Covid-19.
      On the other hand, several experimental data discussed above suggest the potential utility of ARBs, particularly to limit lung inflammation during viral invasion. [
      • Gurwitz D.
      Angiotensin receptor blockers as tentative SARS-CoV-2 therapeutics.
      ,
      • Verdecchia P
      • Angeli F
      • Reboldi G
      Angiotensin-converting enzyme inhibitors, angiotensin II receptor blockers and coronavirus.
      It is hoped that the results of the above mentioned NCT04312009 and NCT04311177 trials will answer this question.

      8. Conclusions

      We suggest that ACE2 down-regulation induced by the cell entry of SARS-CoV, NL63 and SARS-CoV-2 may be particularly detrimental in subjects with pre-existing ACE2 deficiency. Some degree of ACE2 deficiency has been associated with a variety of conditions including older age, hypertension, diabetes and cardiovascular disease, which also characterize people more likely to be infected and to present more severe complications. In a setting of enhanced ACE2 deficiency produced by the viral invasion, the marked dysregulation between the ‘adverse’ ACE→Angiotensin II→AT1 axis and the ‘protective’ ACE2→Angiotensin1-7→Mas axis would contribute to enhance the progression of inflammatory and thrombotic processes. These considerations provide a rationale for investigating the role of therapeutic approaches conceptually linked to ACE2 receptor activity. These include the use of soluble recombinant ACE2, angiotensin1-7, and angiotensin II type 1 receptor blockers, which are currently being evaluated.

      SOURCES OF FUNDING

      Study supported in part by the no-profit Fondazione Umbra Cuore e Ipertensione-ONLUS, Perugia, Italy

      DISCLOSURES

      None

      Declaration of competing interest

      The authors declare they have no conflict of interest.

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